The present invention relates to a method for removing sulfur dioxide from exhaust gas using an aqueous slurry of particulate adsorbent, preferably activated carbon.
Most available fossil fuels contain substantial quantities of sulfur. When burned for power generation, space heating, transportation and industrial purposes or the like, such sulfur is oxidized to sulfur dioxide gas and expelled into the atmosphere in an exhaust flue gas. For example, combustion of coal containing about 3.5% sulfur produces an exhaust flue gas containing sulfur dioxide at a concentration on the order of 3000 ppm. Recently, environmental considerations have prompted legislation to limit the maximum amount of sulfur dioxide in exhaust flue gases to an amount on the order of 200 to 500 ppm. This has caused many users of fossil fuels to change from the relatively inexpensive and readily available high-sulfur content fuels to more expensive and scarcer forms of fuel, such as low-sulfur coal, oil and natural gas. Shortages of these alternate fuels require the use of the higher sulfur fuels, particularly coal. Thus, there is a need for economical and effective techniques for removing sulfur dioxide from the exhaust gas burning such sulfur-containing fuels.
A number of processes have been proposed for the removal of sulfur dioxide from flue gases. In one technique, dry limestone is injected into the exhaust gas breeching. Other processes utilize wet scrubbing with alkali or alkaline earth metals. However, the operating costs for these processes are relatively high. Furthermore, they generally produce voluminous quantities of by-product sludge with no commercial value and with disposal presenting a difficult problem. Also, serious operating difficulties are caused by precipitation of solid matter formed by the sulfur removal reactions onto the equipment.
One theoretical approach has been suggested in the AICHE Symposium Series, No. 134, Vol. 69, pages 71-75, by J. T. Seburn and A. J. Engel entitled "Sorption of Sulfur Dioxide by Suspension of Activated Carbon in Water." This article suggests that gas mixtures containing sulfur dioxide may be bubbled through suspensions of activated carbon in water in a strictly batch operation. It further suggests that the sulfur dioxide can be converted to sulfur trioxide on the carbon surface if the slurry is exposed to oxygen. However, there is no disclosure of converting this simple laboratory experiment to a continuous operation in which the activated carbon is recycled in a closed loop in the system. Also, it is apparent that the system of sparging the gas would cause too much of a pressure drop to be practical for the massive volumes of flue gas emitted from a power plant or the like. Furthermore, there is no practical disclosure of a technique for forming sulfuric acid at sufficient concentrations to be marketable.